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Abstract:

A fuel cell includes a solid electrolyte layer containing Zr; an
intermediate layer containing CeO2 solid solution having a
rare-earth element excluding Ce; an air electrode layer containing Sr,
the intermediate layer and the air electrode layer being stacked in this
order on one surface of the solid electrolyte layer; and a fuel electrode
layer on another surface of the solid electrolyte layer which is opposite
to the one surface. A value obtained by dividing a content of the
rare-earth element excluding Ce by a content of Zr is equal to or less
than 0.05 at a site of the solid electrolyte layer, the site being 1
μm away from an interface between the solid electrolyte layer and the
intermediate layer.

Claims:

1. A fuel cell, comprising: a solid electrolyte layer containing Zr; an
intermediate layer containing CeO2 solid solution having a
rare-earth element excluding Ce; an air electrode layer containing Sr,
the intermediate layer and the air electrode layer being stacked in this
order on one surface of the solid electrolyte layer; and a fuel electrode
layer on another surface of the solid electrolyte layer which is opposite
to the one surface, wherein a value obtained by dividing a content of the
rare-earth element excluding Ce by a content of Zr is equal to or less
than 0.05 at a site of the solid electrolyte layer, the site being 1
μm away from an interface between the solid electrolyte layer and the
intermediate layer.

2. The fuel cell according to claim 1, wherein a value obtained by
dividing a content of the rare-earth element excluding Ce by a content of
Zr is equal to or less than 0.1 at a site of the solid electrolyte layer,
the site being 0.5 μm away from an interface between the solid
electrolyte layer and the intermediate layer.

3. The fuel cell according to claim 1, wherein the intermediate layer
comprises a first layer located on a surface of the solid electrolyte
layer and a second layer formed on the first layer and located on a
surface of the air electrode layer, and the first layer is denser than
the second layer.

4. A fuel cell, comprising: a solid electrolyte layer containing Zr and
Y; an intermediate layer containing CeO2 solid solution having a
rare-earth element excluding Ce; an air electrode layer containing Sr,
the intermediate layer and the air electrode layer being stacked in this
order on one surface of the solid electrolyte layer; and a fuel electrode
layer on another surface of the solid electrolyte layer which is opposite
to the one surface, wherein a value obtained by dividing a maximum
content of Y in a site of the solid electrolyte layer within 1 μm from
an interface thereof with the intermediate layer by a content of Zr in
the site where the maximum content of Y is detected is equal to or less
than 0.25.

5. The fuel cell according to claim 4, wherein a value obtained by
dividing a content of the rare-earth element excluding Ce by a content of
Zr is equal to or less than 0.1 at a site of the solid electrolyte layer,
the site being 0.5 μm away from the interface between the solid
electrolyte layer and the intermediate layer.

6. The fuel cell according to claim 4, wherein the intermediate layer
comprises a first layer located on a surface of the solid electrolyte
layer and a second layer formed on the first layer and located on a
surface of the air electrode layer, and the first layer is denser than
the second layer.

7. The fuel cell according to claim 3, wherein a thickness of the first
layer is in a range of 0.5 to 10 μm, and a thickness of the second
layer is in a range of 1 to 20 μm.

8. A cell stack, comprising: a plurality of fuel cells according to claim
1, wherein the plurality of fuel cells are electrically connected in
series to each other.

9. A fuel cell module, comprising: the cell stack according to claim 8;
and a housing configured to receive the cell stack therein.

10. A fuel cell device, comprising: the fuel cell module according to
claim 9; an auxiliary device configured to operate the cell stack; and an
exterior case configured to receive the fuel cell module and the
auxiliary device therein.

11. The fuel cell according to claim 6, wherein a thickness of the first
layer is in a range of 0.5 to 10 μm, and a thickness of the second
layer is in a range of 1 to 20 μm.

12. A cell stack, comprising: a plurality of fuel cells according to
claim 4, wherein the plurality of fuel cells are electrically connected
in series to each other.

13. A fuel cell module, comprising: the cell stack according to claim 12;
and a housing configured to receive the cell stack therein.

14. A fuel cell device, comprising: the fuel cell module according to
claim 13; an auxiliary device configured to operate the cell stack; and
an exterior case configured to receive the fuel cell module and the
auxiliary device therein.

Description:

TECHNICAL FIELD

[0001] The present invention relates to a fuel cell, a cell stack in which
a plurality of fuel cells are arranged, a fuel cell module in which the
cell stack is contained in a housing, and a fuel cell device including
the fuel cell module.

BACKGROUND ART

[0002] Recently, various fuel cell modules in which a cell stack composed
of a plurality of fuel cells capable of acquiring electric power using
fuel gas (hydrogen-containing gas) and air (oxygen-containing gas) is
contained in a housing, or various fuel cell devices in which the fuel
cell modules are contained in an exterior case have been proposed as
next-generation energy (for example, Patent Literature 1).

[0003] In such fuel cells, a fuel electrode layer containing Ni and
ZrO2 solid solution having a rare-earth element, a solid electrolyte
layer containing ZrO2 solid solution having a rare-earth element,
and an air electrode layer formed of Sr-containing perovskite composite
oxide are stacked in this order on a conductive support substrate.

[0004] However, since Sr contained in the air electrode layer diffuses
into the solid electrolyte layer or Zr contained in the solid electrolyte
layer diffuses into the air electrode layer in the course of
manufacturing the fuel cells or generating electric power, there is a
problem in that a high-resistance reaction product is formed and thus
power generation performance of the fuel cells is deteriorated in power
generation over a long period of time.

[0005] Therefore, the applicant proposes a fuel cell in which two
intermediate layers formed of CeO2 solid solution having a
rare-earth element excluding Ce is disposed between the solid electrolyte
layer and the air electrode layer, in order to suppress the diffusion of
Sr contained in the air electrode layer into the solid electrolyte layer
or the diffusion of Zr contained in the solid electrolyte layer into the
air electrode layer and to suppress the formation of a high-resistance
reaction product (for example, see Patent Literatures 2 to 4).

[0006] In the course of manufacturing a fuel cell or generating electric
power using the fuel cell in which two intermediate layers formed of
CeO2 solid solution having a rare-earth element is disposed between
the solid electrolyte layer and the air electrode layer, as described in
Patent Literatures 2 to 4, when a large amount of rare-earth element
excluding Ce in CeO2 solid solution contained in the intermediate
layer is present in the solid electrolyte layer (particularly, around the
interface of the solid electrolyte layer with the intermediate layer),
ionic conductivity in a low temperature range (550° C. to
650° C.) around the interface of the solid electrolyte layer with
the intermediate layer is particularly lowered, thereby causing a problem
in that the power generation performance at a low temperature is lowered.
[0007] Patent Literature 1: Japanese Unexamined Patent Publication JP-A
2007-59377 [0008] Patent Literature 2: Japanese Unexamined Patent
Publication JP-A 2008-78126 [0009] Patent Literature 3: Japanese
Unexamined Patent Publication JP-A 2008-226653 [0010] Patent Literature
4: Japanese Unexamined Patent Publication JP-A 2008-226654

SUMMARY OF INVENTION

[0011] The invention is made in consideration of the above-mentioned
problems and relates to a fuel cell, a cell stack, a fuel cell module,
and a fuel cell device, which have improved power generation performance
at a low temperature.

[0012] A fuel cell according to an embodiment of the invention includes a
solid electrolyte layer containing Zr; an intermediate layer containing
CeO2 solid solution having a rare-earth element excluding Ce; an air
electrode layer containing Sr, the intermediate layer and the air
electrode layer being stacked in this order on one surface of the solid
electrolyte layer; and a fuel electrode layer on another surface of the
solid electrolyte layer which is opposite to the one surface, wherein a
value obtained by dividing a content of the rare-earth element excluding
Ce by a content of Zr is equal to or less than 0.05 at a site of the
solid electrolyte layer, the site being 1 μm away from an interface
between the solid electrolyte layer and the intermediate layer.

[0013] A fuel cell according to another embodiment of the invention
includes a solid electrolyte layer containing Zr and Y; an intermediate
layer containing CeO2 solid solution having a rare-earth element
excluding Ce; an air electrode layer containing Sr, the intermediate
layer and the air electrode layer being stacked in this order on one
surface of the solid electrolyte layer; and a fuel electrode layer on
another surface of the solid electrolyte layer which is opposite to the
one surface, wherein a value obtained by dividing a maximum content of Y
in a site of the solid electrolyte layer within 1 μm from an interface
thereof with the intermediate layer by a content of Zr is equal to or
less than 0.25.

[0014] In the fuel cell, it is possible to suppress lowering of the ionic
conductivity in a low temperature range around the interface of the solid
electrolyte layer with the intermediate layer and thus to provide a fuel
cell with improved power generation performance.

[0015] A cell stack according to an embodiment of the invention includes a
plurality of fuel cells mentioned above, wherein the plurality of fuel
cells are electrically connected in series to each other, and therefore
it is possible to provide a cell stack with improved power generation
performance at a low temperature.

[0016] A fuel cell module according to an embodiment of the invention
includes the cell stack mentioned above and a housing configured to
receive the cell stack therein, and therefore it is possible to provide a
fuel cell module with improved power generation performance at a low
temperature.

[0017] A fuel cell device according to an embodiment of the invention
includes the fuel cell module mentioned above, an auxiliary device
configured to operate the cell stack, and an exterior case configured to
receive the fuel cell module and the auxiliary device therein, and
therefore it is possible to provide a fuel cell device with improved
power generation performance at a low temperature.

Advantageous Effects of Invention

[0018] According to the embodiments of the invention, it is possible to
improve power generation performance at a low temperature.

BRIEF DESCRIPTION OF DRAWINGS

[0019] FIG. 1 is a diagram illustrating an embodiment of a fuel cell
according to the invention, where FIG. 1(a) is a cross-sectional view and
FIG. 1(b) is a perspective view of a fuel cell of which a part is
exploded;

[0020] FIG. 2 is an enlarged cross-sectional view illustrating a part of a
power generation part in the embodiment of the fuel cell according to the
invention;

[0021] FIG. 3 is a perspective view illustrating the appearance of an
embodiment of a fuel cell module including the fuel cell according to the
invention; and

[0022] FIG. 4 is an exploded perspective view illustrating an embodiment
of a fuel cell device according to the invention of which parts are
omitted.

DESCRIPTION OF EMBODIMENTS

[0023] Hereinafter, embodiments of the invention will be described with
reference to the accompanying drawings.

[0024] FIG. 1(a) is a cross-sectional view of a hollow panel-shaped fuel
cell 10, and FIG. 1(b) is a perspective view of the fuel cell of which a
part is exploded. In both drawings, members of the fuel cell 10 are
partially enlarged or the like. FIG. 2 is an enlarged cross-sectional
view illustrating a part of a power generation part of the fuel cell 10
according to the invention.

[0025] The fuel cell 10 includes a conductive support substrate 3 having
an elliptic cylinder shape as a whole. A plurality of fuel gas flow
channels 5 are formed in the length direction at a predetermined interval
in the conductive support substrate 3. The fuel cell 10 has a structure
in which various members are formed on the conductive support substrate
3.

[0026] The conductive support substrate 3 includes a flat part n and
arc-like parts m at both ends of the flat part n, as can be understood
from the shape shown in FIG. 1(a). Both surfaces of the flat part n are
substantially parallel to each other. A fuel electrode layer 7 is
disposed to cover one surface (the lower surface) of the flat part n and
both arc-like parts m and a solid electrolyte layer 9 which is dense is
stacked to cover the fuel electrode layer 7. An air electrode layer 1 is
stacked on the solid electrolyte layer 9 to face the fuel electrode layer
7 with an intermediate layer 4 interposed therebetween. An interconnector
2 is formed on the other surface of the flat part n on which the fuel
electrode layer 7 and the solid electrolyte layer 9 are not stacked. As
can be clearly seen from FIGS. 1(a) and 1(b), the fuel electrode layer 7
and the solid electrolyte layer 9 extend to both sides of the
interconnector 2 via the arc-like parts m at both ends so as not to
expose the surface of the conductive support substrate 3 to the outside.

[0027] Here, in the fuel cell 10, the part of the fuel electrode layer 7
facing (opposing) the air electrode layer 1 serves as a fuel electrode to
generate electric power. That is, by causing oxygen-containing gas such
as air to flow outside the air electrode layer 1, causing fuel gas
(hydrogen-containing gas) to flow in the gas flow channels 5 in the
conductive support substrate 3, and heating the fuel cell to a
predetermined operation temperature, electric power is generated. Current
generated by this power generation is collected by the interconnector 2
bonded to the conductive support substrate 3. Members constituting the
fuel cell 10 will be sequentially described below.

[0028] The conductive support substrate 3 is preferably formed of, for
example, an iron group metal component and a rare-earth element oxide,
from the requirements that it should be gas-permeable to transmit fuel
gas to the fuel electrode layer 7 and it should be conductive to collect
power through the use of the interconnector 2.

[0029] Examples of the iron group metal component include iron group metal
simple, iron group metal oxide, and alloy or alloy oxide of iron group
metal. More specific examples of the iron group metal include Fe, Ni
(nickel), and Co. In the invention, any thereof can be used, but the iron
group component preferably contains Ni and/or NiO in view of low cost and
stability in the fuel gas. A plurality of iron group metal components may
be included.

[0030] The rare-earth element oxide is used to cause the thermal expansion
coefficient of the conductive support substrate 3 to approach the thermal
expansion coefficient of the solid electrolyte layer 9. A rare-earth
element oxide containing at least one element selected from the group
consisting of Y, Lu (lutetium), Yb, Tm (thulium), Er (erbium), Ho
(holmium), Dy (dysprosium), Gd, Sm, and Pr (praseodymium) is used in
combination with the iron group component. Specific examples of the
rare-earth element oxide include Y2O3, Lu2O3,
Yb2O3, Tm2O3, Er2O3, Ho2O3,
Dy2O3, Gd2O3, Sm2O3, and Pr2O3,
and Y2O3 and Yb2O3 can be preferably used in that
solid solution and reaction with the iron group metal oxide are hardly
caused, the thermal expansion coefficients are substantially equal to the
thermal expansion coefficient of the solid electrolyte layer 9, and they
are low in cost.

[0031] In that the conductivity of the conductive support substrate 3 is
maintained well and the thermal expansion coefficient is caused to
approach that of the solid electrolyte layer 9, the volume ratio after
firing-reduction is preferably in the range of 35:65 to 65:35 in terms of
iron group metal component:rare-earth element oxide (for example,
Ni:Y2O3) (for example, in the range of 65 to 86 mol % in terms
of the mole ratio of iron group metal component/(iron group metal
component+Y)). The conductive support substrate 3 may further contain
other metal components or oxide components in the range in which
necessary characteristics are not damaged.

[0032] Since the conductive support substrate 3 needs to have fuel gas
permeability, the open porosity is preferably equal to or more than 30%
and more preferably in the range of 35% to 50%. The conductivity of the
conductive support substrate 3 is preferably equal to or more than 50
S/cm, more preferably equal to or more than 300 S/cm, and still more
preferably equal to or more than 440 S/cm.

[0033] In the hollow panel-type fuel cell 10 shown in FIG. 1, when the
length (the length in the width direction of the conductive support
substrate 3) of the flat part n of the conductive support substrate 3 is
set to 15 to 35 mm and the length (the length of the arc) of the arc-like
part m is set to 2 to 8 mm, the thickness (the thickness between both
surfaces of the flat part n) of the conductive support substrate 3 is
preferably in the range of 1.5 to 5 mm.

[0034] The fuel electrode layer 7 causes an electrode reaction and is
preferably formed of known porous conductive ceramics. For example, the
fuel electrode layer is formed of ZrO2 solid solution having a
rare-earth element excluding Zr or CeO2 solid solution having a
rare-earth element excluding Ce, and Ni and/or Nio.

[0035] Regarding the content of ZrO2 solid solution having a
rare-earth element excluding Zr or CeO2 solid solution having a
rare-earth element excluding Ce and the content of Ni or NiO in the fuel
electrode layer 7, the volume ratio after firing-reduction is preferably
in the range of 35:65 to 65:35 in terms of the volume ratio of
Ni:ZrO2 solid solution having a rare-earth element excluding Zr
(Ni:YSZ) or CeO2 solid solution having a rare-earth element
excluding Ce. The open porosity of the fuel electrode layer 7 is
preferably equal to or more than 15% and more preferably in the range of
20% to 40%. The thickness thereof is preferably in the range of 1 to 30
μm. For example, it is possible to enhance the power generation
performance by setting the thickness of the fuel electrode layer 7 to the
above-mentioned range and it is possible to suppress the separation due
to a difference in thermal expansion between the solid electrolyte layer
9 and the fuel electrode layer 7 by setting the thickness to the
above-mentioned range.

[0036] In the example shown in FIGS. 1(a) and 1(b), since the fuel
electrode layer 7 extends to both side surfaces of the interconnector 2
but the fuel electrode layer 7 has only to be formed to exist at a
position facing the air electrode layer 1, the fuel electrode layer 7 may
be formed, for example, on only the flat part n on the side where the air
electrode layer 1 is formed. The interconnector 2 may be formed directly
on the flat part n of the conductive support substrate 3 on the side
where the solid electrolyte layer 9 is not formed. In this case, it is
possible to suppress the potential drop between the interconnector 2 and
the conductive support substrate 3.

[0037] The solid electrolyte layer 9 formed on the fuel electrode layer 7
is preferably formed of ceramics which is a dense material including
partially-stabilized or stabilized ZrO2 containing 3 to 15 mol % of
a rare-earth element such as Y (yttrium), Sc (scandium), and Yb
(ytterbium). In view of cost, Y can be preferably used as the rare-earth
element. In view of prevention of gas permeation, the solid electrolyte
layer 9 is preferably formed of a dense material having a relative
density (pursuant to Archimedes' principle) of equal to or more than 93%
and more preferably equal to or more than 95%. The thickness thereof is
preferably in the range of 3 to 50 μm.

[0038] The air electrode layer 1 is preferably formed of conductive
ceramics composed of so-called ABO3 type perovskite composite oxide.
The perovskite composite oxide is preferably transition metal perovskite
type oxide, particularly, at least one of LaMnO3-based oxide,
LaFeO3-based oxide, and LaCoO3-based oxide in which Sr and La
(lanthanum) coexist in the A site. In view of high electric conductivity
at an operation temperature of about 600° C. to 1000° C.,
LaCoO3-based oxide can be preferably used. In the perovskite
composite oxide in which Sr and La coexist, Fe (iron) or Mn (manganese)
along with Co may exist in the B site.

[0039] The air electrode layer 1 needs to have gas permeability and the
conductive ceramics (perovskite oxide) forming the air electrode layer 1
preferably has an open porosity of equal to or more than 20% and more
preferably in the range of 30% to 50%. The thickness of the air electrode
layer 1 is preferably in the range of 30 to 100 μm in view of power
collection.

[0040] The interconnector 2 is preferably formed of conductive ceramics,
but needs to have reduction resistance and oxidation resistance since it
comes in contact with fuel gas (hydrogen gas and oxygen-containing gas
(air). Accordingly, lanthanum chromite-based perovskite composite oxide
(LaCrO3-based oxide) is generally used as the conductive ceramics
having reduction resistance and oxidation resistance. In order to prevent
leakage of the fuel gas passing through the inside of the conductive
support substrate 3 and the oxygen-containing gas passing through the
outside of the conductive support substrate 3, the conductive ceramics
has to be dense and preferably has a relative density of equal to or more
than 93% and more preferably a relative density of equal to or more than
95%.

[0041] The thickness of the interconnector 2 is preferably in the range of
3 to 200 μm, in view of prevention of leakage of gas and suppression
of the excessive increase in electric resistance. By setting the
thickness to the above-mentioned range, the leakage of gas is hardly
caused and the electric resistance is not excessively high, thereby
enhancing the power collecting function.

[0042] A layer 8 having a composition similar to that of the fuel
electrode layer 7 may be formed between the interconnector 2 and the
conductive support substrate 3 so as to reduce the difference in thermal
expansion coefficient between the interconnector 2 and the conductive
support substrate 3. In FIGS. 1(a) and 1(b), the layer 8 having a
composition similar to that of the fuel electrode layer 7 is formed
between the interconnector 2 and the conductive support substrate 3.

[0043] It is preferable that a P-type semiconductor layer 6 is formed on
the outer surface (top surface) of the interconnector 2. By connecting a
power collecting member to the interconnector 2 via the P-type
semiconductor layer 6, the contact thereof is an ohmic contact to reduce
the potential drop, thereby effectively avoiding the decrease in power
collecting performance.

[0044] For example, a layer formed of transition metal perovskite type
oxide can be used as the P-type semiconductor layer 6. Specifically, a
P-type semiconductor ceramics including a material having larger electron
conductivity than that of LaCrO3-based oxide forming the
interconnector 2, for example, at least one of LaMnO3-based oxide,
LaFeO3-based oxide, and LaCoO3-based oxide in which Mn, Fe, Co,
and the like coexist in the B site, can be used. The thickness of the
P-type semiconductor layer 6 is preferably in the range of 30 to 100
μm.

[0045] An intermediate layer 4 including CeO2 solid solution having a
rare-earth element excluding Ce is formed on the surface of the solid
electrolyte layer 9. Here, the intermediate layer 4 preferably includes a
first layer 4a located on the side of the solid electrolyte layer 9 and a
second layer 4b formed on the first layer 4a and located on the side of
the air electrode layer 1.

[0046] By forming the intermediate layer 4 including CeO2 solid
solution having a rare-earth element excluding Ce between the solid
electrolyte layer 9 and the air electrode layer 1, it is possible to
suppress the diffusion of Zr which is a component of the solid
electrolyte layer 9 into the air electrode layer 1, it is possible to
suppress the diffusion of Sr or the like which are components of the air
electrode layer 1 into the solid electrolyte layer 9, it is possible to
suppress the production of a reaction product (reaction layer) having
high electric resistance through the reaction thereof, and it is possible
to suppress the deterioration in power generation performance at a low
temperature of the fuel cell 10 during long-term power generation.

[0047] A value obtained by dividing a content of the rare-earth element
excluding Ce at a site of the solid electrolyte layer 9, the site being 1
μm away from an interface between the solid electrolyte layer 9 and
the intermediate layer 4 (the first layer 4a) by a content of Zr at a
site of the solid electrolyte layer 9, the site being 1 μm away from
the interface between the solid electrolyte layer 9 and the intermediate
layer 4 (the first layer 4a) is set to be equal to or less than 0.05.

[0048] The content of the rare-earth element excluding Ce in the
intermediate layer 4 (the first layer 4a and the second layer 4b), the
content of the rare-earth element excluding Ce and the content of Zr in
the solid electrolyte layer 9, and the content of Sr in the air electrode
layer 1 can be obtained through the use of quantitative analysis using
scanning transmission electron microscope-energy dispersive X-ray
spectroscopy (STEM-EDS).

[0049] Specifically, a sample is produced using an FIB (Focused Ion
Beam)-microsampling method so as to include the air electrode layer 1,
the intermediate layer 4, and the solid electrolyte layer 9 of the fuel
cell 10, and quantitative analysis using STEM-EDS is performed on the
sample. The number of samples may be one or more.

[0050] The contents of Zr, Sr, and Ce are obtained through the
quantitative analysis. When the interface between the solid electrolyte
layer 9 and the intermediate layer 4 (the first layer 4a) is determined
by the quantitative analysis, the site satisfying Ce/(Zr--Sr)=1 is
determined as the interface between the solid electrolyte layer 9 and the
intermediate layer 4 (the first layer 4a). As a method of determining the
interface not using the quantitative analysis, a fuel cell is cut and the
section thereof is surface-analyzed with an X-ray micro analyzer (EPMA:
Electron Probe Micro Analyzer) to confirm the interface. When it is
intended to set the value obtained by dividing the content of the
rare-earth element excluding Ce by the content of Zr to be equal to or
less than 0.05 at the site of the solid electrolyte layer 9, the site
being 1 μm away from the interface between the solid electrolyte layer
9 and the intermediate layer 4 (the first layer 4a), the decrease in
ionic conductivity in a low temperature range around the interface of the
solid electrolyte layer 9 with the intermediate layer 4 can be prevented
by reducing the content of the rare-earth element excluding Ce in the
intermediate layer 4 (or by causing the intermediate layer 4 not to
contain the rare-earth element excluding Ce), thereby providing a fuel
cell 10 with improved power generation performance at a low temperature.

[0051] More preferably, the value obtained by dividing the content of the
rare-earth element excluding Ce at the site of the solid electrolyte
layer 9, the site being 0.5 μm away from the interface between the
solid electrolyte layer 9 and the intermediate layer 4 (the first layer
4a) by the content of Zr at the site of the solid electrolyte layer 9,
the site being 0.5 μm away from the interface between the solid
electrolyte layer 9 and the intermediate layer 4 (the first layer 4a) is
set to be equal to or less than 0.1. Accordingly, the decrease in ionic
conductivity in a low temperature range around the interface of the solid
electrolyte layer 9 with the intermediate layer 4 can be further
prevented, thereby providing a fuel cell 10 with improved power
generation performance.

[0052] In producing the second layer 4b of the intermediate layer 4, the
source powder preferably has a composition expressed, for example, by
(CeO2)1-x(REO1.5)x (where RE represents at least one
of Sm, Y, Yb, and Gd and x is a number satisfying 0<x≦0.3).

[0053] Particularly, CeO2 solid solution having Sm or Gd can be
preferably used and the source powder preferably has a composition
expressed by (CeO2)1-x(SmO1.5)x or
(CeO2)1-x(GdO1.5)x (where x is a number satisfying
0<x≦0.3). In view of a decrease in electric resistance,
CeO2 solid solution having 10 to 20 mol % SmO1.5 or GdO1.5
can be preferably used.

[0054] On the other hand, in producing the first layer 4a of the
intermediate layer 4, the first layer 4a and the second layer 4b may be
formed of the same source powder, but CeO2 not containing a
rare-earth element can be preferably used as the source powder, in view
of the effective suppression of diffusion of the rare-earth element
excluding Ce contained in the second layer 4b or CeO2 solid solution
having a rare-earth element excluding Ce into the solid electrolyte layer
9. That is, in the fuel cell 10, the rare-earth element excluding Ce
contained in the second layer 4b or CeO2 solid solution having the
rare-earth element excluding Ce diffuses into the first layer 4a during
production or power generation. Accordingly, when CeO2 solid
solution having no rare-earth element is used as the source powder to
produce the first layer 4a, the first layer 4a includes CeO2 solid
solution having the rare-earth element and CeO2 solid solution
having no rare-earth element. For this reason, in this case, in the
course of manufacturing the fuel cell 10 or generating electric power,
the content of the rare-earth element excluding Ce contained in the first
layer 4a can be made to be smaller than the content of the rare-earth
element excluding Ce contained in the second layer 4b. Accordingly, it is
possible to suppress the particular decrease in ionic conductivity in a
low temperature range around the interface of the solid electrolyte layer
9 with the intermediate layer 4 and thus to provide a fuel cell 10 with
improved power generation performance at a low temperature.

[0055] Since the first layer 4a and the second layer 4b each contain
CeO2, it is possible to improve the adhesion strength between the
first layer 4a and the second layer 4b and to reduce the thermal
expansion coefficient between the first layer 4a and the second layer 4b.
Accordingly, since the thermal expansion coefficient of the intermediate
layer 4 can be made close to the thermal expansion coefficient of the
solid electrolyte layer 9, it is possible to suppress generation of
cracks or separation due to the difference in thermal expansion.

[0056] It is preferable that the intermediate layer 4 includes the first
layer 4a and the second layer 4b and the first layer 4a is denser than
the second layer 4b.

[0057] Accordingly, even when Sr constituting the air electrode layer 1
permeates the second layer 4b, it is possible to suppress the diffusion
of Sr or the like by the use of the denser first layer 4a and to suppress
the diffusion of Sr or the like into the solid electrolyte layer 9.
Accordingly, in the solid electrolyte layer 9, it is possible to prevent
formation of a reaction layer having high electric resistance due to a
reaction between Zr in the solid electrolyte layer 9 and Sr or the like
in the air electrode layer 1.

[0058] The first layer 4a and the second layer 4b, oxide of another
rare-earth element (for example, Y2O3 and Yb2O3) may
be added to the source powder in order to enhance the effect of
suppressing the diffusion of Zr in the solid electrolyte layer 9 and to
enhance the effect of suppressing the formation of a reaction product
between Zr in the solid electrolyte layer 9 and Sr or the like.

[0059] Here, it is preferable that the solid electrolyte layer 9 and the
first layer 4a are formed by co-firing. That is, the second layer 4b is
preferably formed through a separate process after the solid electrolyte
layer 9 and the first layer 4a are formed by co-firing.

[0060] In this method of producing a fuel cell 10, since the solid
electrolyte layer 9 and the first layer 4a are formed at a high
temperature by co-firing as described later, Zr of the solid electrolyte
layer 9 diffuses into the first layer 4a, the solid electrolyte layer 9
and the first layer 4a can be strongly bonded to each other, separation
of the first layer 4a from the solid electrolyte layer 9 can be
suppressed, and the first layer 4a can be made to be dense.

[0061] The second layer 4b can be made to have a low density by forming
the second layer on the surface of the first layer 4a through a process
separated from the co-firing. Therefore, for example, when the air
electrode layer 1 is formed after the second layer 4b is formed, it is
possible to enhance the adhesion strength by an anchor effect.
Accordingly, it is possible to suppress the separation of the air
electrode layer 1 from the second layer 4b and to suppress the decrease
in power generation performance of the fuel cell 10 in long-term power
generation. Since the contact area between the second layer 4b and the
air electrode layer 1 can increase, it may be possible to lower reaction
resistance.

[0062] It is possible to lower the rigidity of the second layer 4b by
forming the second layer 4b with a low density, it is possible to reduce
a thermal stress when the thermal stress is generated due to the
difference in thermal expansion from the air electrode layer 1, it is
possible to suppress the separation of the air electrode layer 1 from the
second layer 4b, and it is possible to suppress the decrease in power
generation performance of the fuel cell 10 in long-term power generation.

[0063] The second layer 4b has only to have a density lower than that of
the first layer 4a and it is not limited to making the second layer 4b
denser in order to suppress the diffusion of Sr or the like in the air
electrode layer 1 into the solid electrolyte layer 9. Here, it is
preferable that the density is appropriately adjusted to form the second
layer 4b so as to strongly bond the second layer 4b and the air electrode
layer 1 to each other.

[0064] The second layer 4b of the intermediate layer 4 may include a
plurality of layers. Therefore, for example, the second layer 4b may
include two layers and the intermediate layer 4 may include three layers
as a whole, or the intermediate layer may include a larger number of
layers.

[0065] Here, when the second layer 4b includes a plurality of layers, it
is preferable that the layer bonded to the air electrode layer 1 is
strongly bonded thereto. Accordingly, the second layer 4b can be
appropriately formed by separately forming the layer bonded to the air
electrode layer 1 after sequentially forming the layers constituting the
second layer 4b.

[0066] By causing the second layer 4b disposed not to be bonded to the air
electrode layer 1 out of the plurality of layers to be denser, it is
possible to reduce the grain boundaries or surface area in the second
layer 4b and to further suppress the diffusion of Sr or the like
contained in the air electrode layer 1 into the solid electrolyte layer
9. In causing the second layer 4b to be denser, the second layer 4b can
be made to be denser by appropriately changing the thermal process
temperature or the thermal process time of the second layer 4b on the
basis of the particle size of the source material of the second layer 4b.

[0067] The second layer 4b preferably has a density lower than that of the
first layer 4a and thus the second layer is preferably fired at a
temperature lower than, for example, the co-firing temperature of the
first layer 4a and the solid electrolyte layer 9.

[0068] By co-firing the solid electrolyte layer 9 and the first layer 4a
and then firing the second layer 4b on the first layer 4a at a
temperature lower than the co-firing temperature of the solid electrolyte
layer 9 and the first layer 4a, it is possible to suppress the diffusion
of Zr in the solid electrolyte layer 9 into the second layer 4b.
Accordingly, the second layer 4b does not contain Zr and it is possible
to suppress the formation of a reaction layer having high electric
resistance in the air electrode layer 1 disposed on the second layer 4b.

[0069] By firing and forming the second layer 4b at a temperature lower
than the co-firing temperature of the solid electrolyte layer 9 and the
first layer 4a, it is possible to lower the denseness of the second layer
4b. Accordingly, it is possible to strongly bond the second layer 4b and
the air electrode layer 1 to each other.

[0070] When firing the second layer 4b at a temperature lower than the
co-firing temperature of the solid electrolyte layer 9 and the first
layer 4a, specifically, the second layer is preferably fired at a
temperature lower by 200° C. or higher than the co-firing
temperature of the solid electrolyte layer 9 and the first layer 4a.
Regarding the specific temperature, the second layer 4b is preferably
fired and formed, for example, at 1100° C. to 1300° C.

[0071] It is preferable that the thickness of the first layer 4a be in the
range of 0.5 to 10 μm and the thickness of the second layer 4b be in
the range of 1 to 20 μm. By setting the thickness of the first layer
4a to be in the range of 0.5 to 10 μm, it is possible to suppress the
diffusion of the rare-earth element excluding Ce contained in the
intermediate layer 4 into the solid electrolyte layer 9. Accordingly, it
is possible to particularly suppress the decrease in ionic conductivity
in the low temperature range around the interface between the solid
electrolyte layer 9 and the first layer 4a and thus to provide a fuel
cell 10 with improved power generation performance at a low temperature.

[0072] By setting the thickness of the first layer 4a to be in the range
of 0.5 to 10 μm, it is possible to allow Zr contained in the solid
electrolyte layer 9 to satisfactorily diffuse into the first layer 4a to
strongly bond the solid electrolyte layer 9 and the first layer 4a to
each other and it is possible to suppress the separation of the first
layer 4a from the solid electrolyte layer 9.

[0073] On the other hand, by setting the thickness of the second layer 4b
to be in the range of 1 to 20 μm, it is possible to enhance the
bonding strength between the first layer 4a and the second layer 4b and
to suppress the separation of the second layer 4b from the first layer
4a. When the thickness of the second layer 4b is larger than 20 μm,
the second layer 4b may be separated from the first layer 4a due to the
difference in thermal expansion from the first layer 4a.

[0074] By setting the thickness of the second layer 4b to be in the range
of 1 to 20 μm, it is possible to reduce the amount of Sr in the air
electrode layer 1 permeating the second layer 4b due to long-term
continuous operation. Accordingly, it is possible to suppress the
diffusion of Sr in the air electrode layer 1 into the solid electrolyte
layer 9, to suppress the decrease in power generation performance of the
fuel cell 10 in the long-term power generation, and thus to provide a
fuel cell 10 with excellent long-term reliability.

[0075] When the intermediate layer 4 containing CeO2 solid solution
having the rare-earth element excluding Ce is formed on the surface of
the solid electrolyte layer 9 containing Zr and Y, the power generation
performance may be low at a high temperature. Although the cause is not
clear, as the result of study of the components of the fuel cell 10, it
is thought that a site (peak part) in which the content of Y in the solid
electrolyte layer 9 is partially high is present in the site f the solid
electrolyte layer 9 within 1 μm from the interface thereof with the
intermediate layer 4 and the presence of the site in which the content of
Y is high is associated with the low power generation performance at a
high temperature.

[0076] Accordingly, the value obtained by dividing the maximum content of
Y in the site of the solid electrolyte layer 9 within 1 μm from the
interface thereof with the intermediate layer 4 (the first layer 4a) by
the content of Zr in the site where the maximum content of Y is detected
is set to be equal to or less than 0.25. As a result, it is possible to
suppress the decrease in power generation performance at a high
temperature.

[0077] The content of Y or the content of Z in the solid electrolyte layer
9 can be obtained through the use of the quantitative analysis using
scanning transmission electron microscope-energy dispersive X-ray
spectroscopy (STEM-EDS).

[0078] A method of manufacturing the above-mentioned hollow panel-type
fuel cell 10 will be described below.

[0079] First, a powder of iron group metal such as Ni or oxide thereof, a
powder of rare-earth element oxide such as Y2O3, an organic
binder, and a solvent are blended to prepare a green body, a conductive
support substrate compact is produced using the green body through the
use of an extrusion molding method, and the resultant is dried. A
calcined body obtained by calcining the conductive support substrate
compact at 900° C. to 1000° C. for 2 to 6 hours may be used
as the conductive support substrate compact.

[0080] Then, for example, a source material of NiO and ZrO2 solid
solution having Y2O3 (YSZ) in accordance with a predetermined
combination composition is weighed and blended thereto. Thereafter, an
organic binder and a solvent are blended to prepare a fuel electrode
layer slurry.

[0081] A toluene, a binder, and a commercially-available dispersant are
added to the ZrO2 powder having the rare-earth element excluding Zr
to form a slurry and this slurry is shaped with a thickness of 7 to 75
μm through the use of a doctor blade method or the like to produce a
sheet-like solid electrolyte layer compact. The fuel electrode layer
slurry is applied onto the sheet-like solid electrolyte layer compact to
form a fuel electrode layer compact and the surface of the fuel electrode
layer compact is stacked on the conductive support substrate compact. The
fuel electrode layer slurry may be applied to a predetermined position of
the conductive support substrate compact and may be dried and then the
solid electrolyte layer compact coated with the fuel electrode layer
slurry may be stacked on the conductive support substrate compact.

[0082] Then, the intermediate layer 4 is formed. In the formation of the
intermediate layer 4, when the first layer 4a and the second layer 4b are
formed of the same source powder, it is preferable that an intermediate
layer compact to be described later be stacked thereon and then the
resultant be co-fired without calcining in advance a stacked body in
which the fuel electrode layer compact and the solid electrolyte layer
compact are stacked on the conductive support substrate compact. When the
first layer 4a and the second layer 4b are formed of different source
powders, it is preferable that the stacked body be calcined in advance
and then the intermediate layer compact to be described later be stacked
thereon.

[0083] For example, a powder of CeO2 not containing a rare-earth
element or a powder of CeO2 solid solution having GdO1.5 or a
powder of CeO2 solid solution having SmO1.5 is wet-crushed to
prepare a source powder for a first layer compact out of the intermediate
layer compact 4. The wet crushing is performed, for example, with a ball
mill using a solvent for 10 to 20 hours.

[0084] Toluene as a solvent is added to the source powder for the first
layer compact of which the degree of aggregation is adjusted to prepare a
first layer slurry and this slurry is applied to the solid electrolyte
layer compact to produce the first layer compact. A sheet-like first
layer compact may be produced and then may be stacked on the solid
electrolyte layer compact.

[0085] Here, when the first layer compact is formed of the CeO2
powder not containing a rare-earth element excluding Ce, it is preferable
that the stacked body in which the fuel electrode layer compact and the
solid electrolyte layer compact are stacked on the conductive support
substrate compact not be calcined in advance. When the first layer
compact is formed of the powder of CeO2 solid solution having
GdO1.5 or the powder of CeO2 solid solution having SmO1.5,
it is preferable that the stacked body in which the fuel electrode layer
compact and the solid electrolyte layer compact are stacked on the
conductive support substrate compact be calcined in advance. Accordingly,
the value obtained by dividing the content of the rare-earth element
excluding Ce by the content of Zr can be set to be equal to or less than
0.05 at the site of the solid electrolyte layer 9, the site being 1 μm
away from the interface between the solid electrolyte layer 9 and the
intermediate layer 4 and it is possible to suppress the decrease in ionic
conductivity in a low temperature range and thus to provide a fuel cell
with improved power generation performance at a low temperature.

[0086] Subsequently, an interconnector material (for example, a
LaCrO3-based oxide powder), an organic binder, and a solvent are
blended to prepare a slurry, this slurry is formed in a sheet shape to
produce an interconnector sheet, and the interconnector sheet is stacked
on the exposed surface of the conductive support substrate compact on
which the solid electrolyte layer compact is not formed, whereby a
stacked compact is produced.

[0087] Then, the stacked compact is subjected to a binder removing process
and is fired at 1400° C. to 1600° C. in the atmosphere
containing oxygen for 2 to 6 hours.

[0088] Subsequently, for example, the powder of CeO2 solid solution
having GdO1.5 or SmO1.5 is heated at 800° C. to
900° C. for 2 to 6 hours, and the resultant is wet-crushed to
adjust the degree of aggregation to the range of 5 to 35, whereby the
source powder for the second layer compact of the intermediate layer 4 is
prepared. The wet crushing is preferably performed, for example, with a
ball mill using a solvent for 10 to 20 hours. The same is true when the
second layer is formed of the powder of CeO2 solid solution having
SmO1.5.

[0089] Toluene as a solvent is added to the source powder for the second
layer compact of which the degree of aggregation is adjusted to prepare a
second layer slurry, the second layer slurry is applied to the surface of
the first layer 4a formed by sintering to produce the second layer
compact, and the second layer compact is fired. In the firing of the
second layer compact, the firing temperature is preferably lower by
200° C. or higher than the firing temperature of the solid
electrolyte layer 9 and the first layer 4a and is more preferably in the
range of 1100° C. to 1300° C. Accordingly, it is possible
to suppress the diffusion of Sr or the like in the air electrode layer 1
into the solid electrolyte layer 9.

[0090] When the second layer 4b includes a plurality of layers, the layers
constituting the second layer 4b can be produced by appropriately
adjusting the production method such as by preparing the source powders
as described above, adding toluene to the source powders to prepare
slurries, and applying and sequentially stacking the slurries, and
independently firing the respective layers.

[0091] Here, in causing the second layer 4b to be denser, the particle
size, the firing temperature, and the firing time, and the like of the
source material for the second layer compact can be appropriately
adjusted. By firing to fix the second layer 4b and the first layer 4a and
then baking the resultant in, the second layer 4b may be made to be
denser. When the second layer 4b is made to be denser, the bonding
strength to the air electrode layer 1 may be lowered. Accordingly, it is
preferable that the baking temperature or the baking time be
appropriately adjusted to strongly bond the second layer 4b and the air
electrode layer 1 to each other. The firing time for fixing the second
layer 4b and the first layer 4a is preferably in the range of 2 to 6
hours.

[0092] Then, a slurry including an air electrode layer material (for
example, LaCoO3-based oxide powder), a solvent, and a pore-forming
agent is applied to the second layer 4b through the use of a dipping
method or the like. A slurry including a P-type semiconductor layer
material (for example, LaCoO3-based oxide powder) and a solvent is
applied to a predetermined position on the interconnector 2 through the
use of a dipping method or the like and the resultant is baked in at
1000° C. to 1300° C. for 2 to 6 hours if necessary, whereby
it is possible to manufacture the hollow panel-type fuel cell 10 having
the structure shown in FIGS. 1(a) and 1(b). Thereafter, it is preferable
that hydrogen gas be made to flow in the fuel cell 10 to perform a
reduction process on the conductive support substrate 3 and the fuel
electrode layer 7. At this time, the reduction process is preferably
performed, for example, 750° C. to 1000° for 5 to 20 hours.

[0093] That is, since the second layer 4b is baked in and then the air
electrode layer 1 is baked in to manufacture the fuel cell 10, it is
possible to suppress the diffusion of the components of the air electrode
layer 1 into the second layer 4b. Accordingly, it is possible to suppress
the diffusion of the components of the air electrode layer 1 into the
solid electrolyte layer 9 just after manufacturing the fuel cell 10.

[0094] In the manufactured fuel cell 10, the value obtained by dividing
the content of the rare-earth element excluding Ce at the site of the
solid electrolyte layer 9, the site being 1 μm away from the interface
between the solid electrolyte layer 9 and the intermediate layer 4 (the
first layer 4a) by the content of Zr at the site of the solid electrolyte
layer 9, the site being 1 μm away from the interface between the solid
electrolyte layer 9 and the intermediate layer 4 (the first layer 4a) can
be set to be equal to or less than 0.05. When the solid electrolyte layer
9 contains Zr and Y, the value obtained by dividing the maximum content
of Y in the site of the solid electrolyte layer 9 within 1 μm from the
interface thereof with the intermediate layer 4 by the content of Zr in
the site where the maximum content of Y is detected can be set to be
equal to or less than 0.25.

[0095] FIG. 3 is a perspective view illustrating an example of a fuel cell
module 11 including a cell stack device 15 including a cell stack 13 and
a housing 12 configured to receive the cell stack device 15 therein, the
cell stack 13 including a plurality of fuel cells 10 according to the
invention and power collecting members (not shown), the plurality of fuel
cells being electrically connected in series with power collecting
members interposed therebetween.

[0096] In order to acquire fuel gas used in the fuel cell 10, a reformer
16 configured to reform a raw material such as natural gas or lamp oil to
generate fuel gas is disposed above the cell stack 13. The fuel gas
generated by the reformer 16 is supplied to a manifold 14 via a gas flow
pipe 17 and is supplied to the fuel gas flow channels 5 formed in the
fuel cells 10 via the manifold 14.

[0097] Since the cell stack 13 includes a plurality of fuel cells 10 with
improved power generation performance at a low temperature which are
electrically connected in series, it is possible to provide a cell stack
13 with improved power generation performance at a low temperature.

[0098] FIG. 3 shows a state where parts (front and rear walls) of the
housing 12 are removed and the cell stack device 15 and the reformer 16
received therein are pulled out backward. Here, in the fuel cell module
11 shown in FIG. 3, the cell stack device 15 can be made to slide and can
be received in the housing 12. The cell stack device 15 may include the
reformer 16.

[0099] An oxygen-containing gas introducing member 18 installed in the
housing 12 is disposed between the manifold 14 and the cell stack 13
juxtaposed thereon in FIG. 3 and supplies oxygen-containing gas to the
lower end of the fuel cells 10 so that the oxygen-containing gas flows in
the side of the fuel cells 10 from the lower end to the upper end with
the flow of the fuel gas. By combusting the fuel gas discharged from the
fuel gas flow channels 5 of the fuel cells 10 and the oxygen-containing
gas at the upper end of the fuel cells 10, it is possible to raise the
temperature of the fuel cells 10 and to accelerate the startup of the
cell stack device 15. By combusting the fuel gas discharged from the fuel
gas flow channels 5 of the fuel cells 10 and the oxygen-containing gas at
the upper end of the fuel cells 10, it is possible to warm the reformer
16 disposed above the fuel cells 10 (the cell stack 13). Accordingly, the
reformer 16 can efficiently perform a reforming reaction.

[0100] Since the fuel cell module 11 according to the invention includes
the cell stack device 15 including the cell stack 13 having the fuel
cells 10 with improved power generation performance at a low temperature,
and the housing 12 configured to receive the cell stack device 15
therein, it is possible to provide a fuel cell module 11 with improved
power generation performance at a low temperature.

[0101] FIG. 4 is an exploded perspective view illustrating an example of a
fuel cell device according to the invention including the fuel cell
module 11 shown in FIG. 3, an auxiliary device (not shown) used to
operate the cell stack 13 (the cell stack device 15), and an exterior
case configured to receive the fuel cell module 11 and the auxiliary
device therein. In FIG. 4, some parts of the configuration are removed.

[0102] In the fuel cell device 19 shown in FIG. 4, an exterior case
including columns 20 and exterior plates 21 is partitioned vertically by
a partition plate 22, the upside space is defined as a module receiving
chamber 23 configured to receive the fuel cell module 11, and the
downside space is defined as an auxiliary device receiving chamber 24
configured to receive auxiliary devices used to operate the fuel cell
module 11. The auxiliary devices received in the auxiliary device
receiving chamber 24 include a water supply unit configured to supply
water to the fuel cell module 11 and a supply unit configured to supply
the fuel gas and air, but the auxiliary devices are not shown.

[0103] An air flow port 25 configured to allow air of the auxiliary device
receiving chamber 24 to flow into the module receiving chamber 23 is
formed in the partition plate 22 and an exhaust port 26 configured to
exhaust air in the module receiving chamber 23 is formed in a part of the
exterior plate 21 constituting the module receiving chamber 23.

[0104] Since the fuel cell device 19 is configured by receiving the fuel
cell module 11 with improved power generation performance at a low
temperature in the module receiving chamber 23 as described above, it is
possible to provide a fuel cell device 19 with improved power generation
performance at a low temperature.

[0105] While the invention has been described in detail, the invention is
not limited to the above-mentioned embodiment but may be modified and
improved in various forms without departing from the concept of the
invention.

[0106] For example, although the hollow panel-type fuel cell including the
conductive support substrate 3 has been described as an example of the
fuel cell 10 according to the invention, a panel-type fuel cell not
including the conductive support substrate 3 or a cylindrical fuel cell
may be used. A fuel cell in which the air electrode layer 1, the solid
electrolyte layer 9, the intermediate layer 4, and the fuel electrode
layer 7 are stacked in this order on the conductive support substrate 3
may be used depending on the configuration of the respective fuel cells.

EXAMPLES

Example 1

[0107] An example where the second layer is formed of a single layer will
be described below.

[0108] First, a green body produced by blending an NiO powder with an
average particle size of 0.5 μm and an Y2O3 powder with an
average particle size of 0.9 μm so that the volume ratio of Ni is 48
vol % and the volume ratio of Y2O3 is 52 vol % in terms of the
volume ratio after firing-reduction and adding an organic binder and a
solvent thereto was molded through the use of an extrusion molding method
and the resultant was dried and degreased to produce a conductive support
substrate compact. In Sample No. 1, the volume ratio of Ni was 45 vol %
and the volume ratio of Y2O3 was 55 vol % in terms of the
volume ratio after firing-reduction of the Y2O3 powder.

[0109] Then, a fuel electrode layer slurry in which an NiO powder with an
average particle size of 0.5 μm, a powder of ZrO2 solid solution
having Y2O3, an organic binder, and a solvent were blended was
produced, the fuel electrode layer slurry was applied to the conductive
support substrate compact through the use of a screen printing method,
and the resultant was dried, whereby a coating layer for the fuel
electrode layer was formed. Then, a solid electrolyte layer sheet with a
thickness of 30 μm was produced through the use of a doctor blade
method using a slurry obtained by blending a powder of ZrO2 solid
solution having 8 mol % yttria (Y2O3) and having a particle
size of 0.8 μm based on a Microtrac method (solid electrolyte layer
source powder), an organic binder, and a solvent. The solid electrolyte
layer sheet was bonded to the coating layer for the fuel electrode layer
and was dried, whereby the stacked compact shown in Table 1 was produced.
The particle size of the ZrO2 powder in Sample No. 3 was 1.0 μm
and the thickness of the solid electrolyte layer sheet in Sample No. 4
was 40 μm.

[0110] Subsequently, Sample Nos. 1 to 11 and Sample Nos. 19 to 23 shown in
Table 1 were calcined at 1000° C. for 3 hours. In Sample Nos. 12
to 14 shown in Table 1, the stacked compacts were not calcined.

[0111] Then, CeO2 was crushed with a vibration mill or a ball mill
using isopropyl alcohol (IPA) as a solvent, whereby a source powder for
the first layer compact was obtained. A composite oxide including 85 mol
% of CeO2 and 15 mol % of any one of other rare-earth element oxides
(SmO1.5, YO1.5, YbO1.5, and GdO1.5) was crushed with
a vibration mill or a ball mill using isopropyl alcohol (IPA) as a
solvent, the resultant was calcined at 900° C. for 4 hours, and
the resultant was crushed again with a ball mill to adjust the degree of
aggregation, whereby a source powder for the first layer compact was
obtained.

[0112] Subsequently, a first layer slurry obtained by adding an
acryl-based binder and toluene to the source powders for the first layer
compact and blending the resultant was applied to the solid electrolyte
layer calcined body of the obtained stacked calcined body or the solid
electrolyte layer compact of the stacked compact through the use of a
screen printing method, whereby a first layer compact was produced.

[0113] Subsequently, an interconnector slurry in which an
LaCrO3-based oxide, an organic binder, and a solvent were blended
was prepared, was stacked on the exposed conductive support substrate
calcined body or conductive support substrate compact not having the
solid electrolyte layer calcined body or solid electrolyte layer compact
formed thereon, and was fired in the atmosphere at 1510° C. for 3
hours.

[0114] Then, a composite oxide including 85 mol % of CeO2 and 15 mol
% of any one of other rare-earth element oxides (SmO1.5, YO1.5,
YbO1.5, and GdO1.5) was crushed with a vibration mill or a ball
mill using isopropyl alcohol (IPA) as a solvent, the resultant was
calcined at 900° C. for 4 hours, and the resultant was crushed
again with a ball mill to adjust the degree of aggregation, whereby a
source powder for the second layer compact was obtained. A second layer
slurry prepared by adding an acryl-based binder and toluene to the source
powder for the second layer compact and blending the resultant was
applied to the surface of the first layer 4a formed by firing through the
use of a screen printing method to form a second layer compact film, and
the resultant was fired at the temperature shown in Table 1 for 3 hours.

[0115] Sample Nos. 15 to 18 shown in Table 1 were formed by firing the
stacked compact without forming the first layer, stacking only the second
layer, and firing the resultant. In Sample No. 19, the second layer was
not formed. In Sample Nos. 1 to 14 and Sample Nos. 20 to 23 having the
first layer and the second layer formed thereon, the first layer was
denser than the second layer by firing the second layer at a temperature
lower than the firing temperature of the first layer. The denseness of
the first layer and the second layer can be evaluated using the relative
density based on an Archimedes' method and the relative density of the
first layer was set to be higher than the relative density of the second
layer.

[0116] Thereafter, a broken-out section was observed by the use of a
scanning electron microscope and the separation of the first layer and
the solid electrolyte layer was checked. The thicknesses of the first
layer and the second layer were measured and described in Table 1.

[0117] Regarding the fixing strength between the second layer and the
solid electrolyte layer or the first layer, the absence of fixing
strength was determined when the separation was caused by rubbing the
resultant with a finger or processing the resultant with an ultrasonic
cleaner, and the presence of fixing strength was determined when the
separation was not caused in any case.

[0118] A mixture solution including a
La0.6Sr0.4Co0.2Fe0.8O3 powder with an average
particle size of 2 μm and isopropyl alcohol was prepared, the mixture
solution was sprayed and applied to the surface of the second layer of a
stacked sintered compact to form an air electrode layer compact, the
resultant was baked in at 1100° C. for 4 hours to form an air
electrode layer, whereby the fuel cell shown in FIGS. 1(a) and 1(b) was
produced.

[0119] The size of the produced fuel cell was 25 mm×200 mm, the
thickness of the conductive support substrate (the distance between both
surfaces of the flat part n) was 2 mm, the open porosity thereof was 35%,
the thickness of the fuel electrode layer was 10 μm, the open porosity
thereof was 24%, the thickness of the air electrode layer was 50 μm,
the porosity thereof is 40%, and the relative density is 97%.

[0120] Hydrogen-containing gas was made to flow in the fuel cell and a
reduction process was performed on the conductive support substrate and
the fuel electrode layer at 850° C. for 10 hours.

[0121] In the obtained fuel cell, the contents of the rare-earth element
excluding Ce in the first layer and the second layer, the contents of the
rare-earth element excluding Ce in the intermediate layer at the sites of
the solid electrolyte layer, the sites being 1 μm and 0.5 μm away
from the interface between the solid electrolyte layer and the
intermediate layer (the first layer or the second layer), and the content
of Zr were measured through the use of the STEM-EDS (quantitative
analysis using scanning transmission electron microscope-energy
dispersive X-ray spectroscopy) quantitative analysis, and the comparison
results of the contents of the rare-earth element excluding Ce in the
first layer and the second layer and the values obtained by dividing the
contents of the rare-earth element excluding Ce in the respective sites
of the solid electrolyte layer by the content of Zr are shown in Table 1.
By causing the sample thickness to be substantially uniform in about 50
nm through the use of the FIB-micro sampling method during the
preparation of the STEM sample, the quantitative error was suppressed.

[0122] In the obtained fuel cell, the diffusion of Zr in the solid
electrolyte layer into the intermediate layer (the first layer and the
second layer) and the diffusion of Sr in the air electrode layer into the
solid electrolyte layer were surface-analyzed with an EPMA (X-ray micro
analyzer) and are described as presence or absence of Zr and Sr in Table
1.

[0123] Regarding the presence and absence of Zr and Sr, the absence was
determined when Zr was not present in the first layer and the second
layer or when Sr was not present in the solid electrolyte layer, whereas
the presence was determined when Zr or Sr was present therein.

[0124] Subsequently, fuel gas was made to flow in the fuel gas flow
channels of the obtained fuel cell, oxygen-containing gas was made to
flow outside the fuel cell, the fuel cell was heated to 600° C. by
the use of an electric furnace, the fuel cell was subjected to a power
generation test for 3 hours under the conditions of a fuel utilization of
75% and a current density of 0.3 A/cm2, and the power generation
performance (voltage) of the fuel cell at 600° C. was checked. The
result was shown in Table 1.

[0125] Thereafter, the fuel cell was heated to 750° C. by the use
of an electric furnace and was allowed to generate power for 1000 hours
under the conditions of a fuel utilization of 75% and a current density
of 0.6 A/cm2. At this time, the voltage after 1000 hours was
measured using the value at a power generation time of 0 as an initial
voltage, and the variation from the initial voltage was calculated as a
decay rate, and the decay rate of the power generation performance was
calculated.

[0126] Regarding the evaluation of deterioration in power generation
performance, the decay rate of less than 0.5% was evaluated as extremely
small, the decay rate of 0.5% to 1% was evaluated as pretty small, the
decay rate of 1% to 3% was evaluated as small, the decay rate of 3% to 5%
was evaluated as large, and the decay rate of equal to or more than 5%
was evaluated as intense. The evaluation results are shown in Table 1.

[0127] From the results shown in Table 1, in Sample Nos. 1 to 12 in which
the intermediate layer was formed using CeO2 solid solution having a
rare-earth element excluding Ce as the source powder of the first layer
and using CeO2 (CeO2 solid solution having no rare-earth
element) as the source powder of the second layer, it can be seen that
the value obtained by dividing the content of the rare-earth element
excluding Ce in the intermediate layer by the content of Zr was equal to
or less than 0.05 at the site of the solid electrolyte layer, the site
being 1 μm away from the interface between the solid electrolyte layer
and the intermediate layer, the value obtained by dividing the content of
the rare-earth element excluding Ce in the intermediate layer by the
content of Zr was equal to or less than 0.1 at the site of the solid
electrolyte layer, the site being 0.5 μm away from the interface
between the solid electrolyte layer and the intermediate layer, the
voltage at 600° C. (mV) was equal to or more than 650 mV, and the
power generation performance at a low temperature was improved.

[0128] In Sample Nos. 20 to 23 produced by forming the intermediate layer
using the powder of CeO2 solid solution having the rare-earth
element excluding Ce as the source powder of the first layer and the
second layer after stacking the solid electrolyte layer compact on the
conductive support substrate compact and calcining the resultant, the
diffusion of the rare-earth element excluding Ce was equal to or more
than 0.06 at the site of the solid electrolyte layer, the site being 1
μm away from the interface between the solid electrolyte layer and the
intermediate layer. However, in Sample Nos. 13 and 14 produced by
stacking the solid electrolyte layer compact on the conductive support
substrate compact and forming the intermediate layer without calcining
using the powder of CeO2 solid solution having the rare-earth
element excluding Ce as the source powder of the first layer and the
second layer, the diffusion of the rare-earth element excluding Ce was
equal to or less than 0.02 at the site of the solid electrolyte layer,
the site being 1 μm away from the interface between the solid
electrolyte layer and the intermediate layer.

[0129] When the first layer and the solid electrolyte layer were co-fired
and the second layer was fired at a temperature lower by 200° C.
or higher than the co-firing temperature of the first layer and the solid
electrolyte layer (Samples Nos. 1 to 9), it can be seen that Zr did not
diffuse into the second layer, the fixing strength of the second layer
was superior, Sr which is the component of the air electrode layer 1 is
not contained in the solid electrolyte layer, and the deterioration in
power generation performance was very small.

[0130] When the first layer and the second layer were made to be thicker
or thinner (Sample Nos. 7 to 9), it can be seen that Zr did not diffuse
into the second layer, the fixing strength of the second layer was
superior, Sr which is the component of the air electrode layer 1 was not
contained in the solid electrolyte layer, and the deterioration in power
generation performance was very small.

[0131] On the other hand, when the first layer was co-fired but the second
layer was fired at a temperature equal to or higher than 1400° C.,
that is, when the firing temperature of the second layer was lower than
the co-firing temperature but the temperature difference was lower than
200° C. (Sample Nos. 10 and 11), it can be seen that Sr which was
the component of the air electrode layer 1 was not contained in the solid
electrolyte layer, the fixing strength of the second layer was superior,
and the voltage at 600° C. (mV) was equal to or higher than 650
mV, but the diffusion of Zr into the second layer appears and thus the
deterioration in power generation performance was large.

[0132] When the second layer was stacked on the sintered body without the
first layer (Sample Nos. 15 to 18) or when the second layer was not
formed (Sample No. 19), it can be seen that the value obtained by
dividing the content of the rare-earth element excluding Ce in the
intermediate layer by the content of Zr was more than 0.05 at the site of
the solid electrolyte layer, the site being 1 μm away from the
interface between the solid electrolyte layer and the intermediate layer,
the value obtained by dividing the content of the rare-earth element
excluding Ce in the intermediate layer by the content of Zr was more than
0.1 at the site of the solid electrolyte layer, the site being 0.5 μm
away from the interface between the solid electrolyte layer and the first
layer, the voltage at 600° C. (mV) was less than 650 mV, the power
generation performance at a low temperature was lowered, and the
deterioration in power generation performance was intense. In Sample Nos.
17 and 18, the fixing strength was present, but this is because the
fixing strength was enhanced due to Zr in the solid electrolyte layer and
Zr diffusing into the second layer.

[0133] In Sample Nos. 20 to 23 produced by forming the intermediate layer
using the powder of CeO2 solid solution having the rare-earth
element excluding Ce as the source powder of the first layer and the
second layer after stacking the solid electrolyte layer compact on the
conductive support substrate compact and calcining the resultant, it can
be seen that Zr did not diffuse into the second layer, the fixing
strength of the second layer was superior, Sr which was the component of
the air electrode layer was not contained in the solid electrolyte layer,
and the deterioration in power generation performance was pretty small,
but that the value obtained by dividing the content of the rare-earth
element excluding Ce in the intermediate layer by the content of Zr was
more than 0.05 at the site of the solid electrolyte layer, the site being
1 μm away from the interface between the solid electrolyte layer and
the intermediate layer, the value obtained by dividing the content of the
rare-earth element excluding Ce in the intermediate layer by the content
of Zr was more than 0.1 at the site of the solid electrolyte layer, the
site being 0.5 μm away from the interface between the solid
electrolyte layer and the first layer, the voltage at 600° C. (mV)
was equal to or less than 600 mV, and the power generation performance at
a low temperature was lowered.

Example 2

[0134] First, a green body produced by blending an NiO powder with an
average particle size of 0.5 μm and an Y2O3 powder with an
average particle size of 0.9 μm so that the volume ratio of Ni is 48
vol % and the volume ratio of Y2O3 is 52 vol % in terms of the
volume ratio after firing-reduction and adding an organic binder and a
solvent thereto was molded through the use of an extrusion molding method
and the resultant was dried and degreased to produce a conductive support
substrate compact. In Sample No. 1, the volume ratio of Ni was 45 vol %
and the volume ratio of Y2O3 was 55 vol % in terms of the
volume ratio after firing-reduction of the Y2O3 powder.

[0135] Then, a fuel electrode layer slurry in which an NiO powder with an
average particle size of 0.5 μm, a powder of ZrO2 solid solution
having Y2O3, an organic binder, and a solvent were blended is
produced, the fuel electrode layer slurry was applied to the conductive
support substrate compact through the use of a screen printing method,
and the resultant was dried, whereby a coating layer for the fuel
electrode layer was formed. Then, a solid electrolyte layer sheet with a
thickness of 30 μm was produced through the use of a doctor blade
method using a slurry obtained by blending a powder of ZrO2 solid
solution having 8 mol % yttria (Y2O3) and having a particle
size of 0.8 μm based on a Microtrac method (solid electrolyte layer
source powder), an organic binder, and a solvent. The solid electrolyte
layer sheet was bonded to the coating layer for the fuel electrode layer
and was dried. The particle size of the ZrO2 powder in Sample No. 3
was 1.0 μm and the thickness of the solid electrolyte layer sheet in
Sample No. 4 was 40 μm.

[0136] Subsequently, in Sample Nos. 1 to 20 shown in Table 1, the compacts
were stacked to produce a stacked compact as described above and the
resultant was calcined at 1000° C. for 3 hours. In Sample Nos. 21
to 23 shown in Table 1, the stacked compact was not calcined.

[0137] Then, CeO2 was crushed with a vibration mill or a ball mill
using isopropyl alcohol (IPA) as a solvent, whereby a source powder for
the first layer compact was obtained. A composite oxide including 85 mol
% of CeO2 and 15 mol % of any one of other rare-earth element oxides
(SmO1.5, YO1.5, YbO1.5, and GdO1.5) was crushed with
a vibration mill or a ball mill using isopropyl alcohol (IPA) as a
solvent, the resultant was calcined at 900° C. for 4 hours, and
the resultant was crushed again with a ball mill to adjust the degree of
aggregation, whereby a source powder for the first layer compact was
obtained.

[0138] Subsequently, a first layer slurry obtained by adding an
acryl-based binder and toluene to the powder and blending the resultant
was applied to the solid electrolyte layer calcined body of the obtained
stacked calcined body through the use of a screen printing method,
whereby a first layer compact was produced.

[0139] Subsequently, an interconnector slurry in which an
LaCrO3-based oxide, an organic binder, and a solvent were blended
was prepared, was stacked on the exposed conductive support substrate
calcined body not having the solid electrolyte layer calcined body formed
thereon, and was fired in the atmosphere at 1510° C. for 3 hours.

[0140] Then, a composite oxide including 85 mol % of CeO2 and 15 mol
% of any one of other rare-earth element oxides (SmO1.5, YO1.5,
YbO1.5, and GdO1.5) was crushed with a vibration mill or a ball
mill using isopropyl alcohol (IPA) as a solvent, the resultant was
calcined at 900° C. for 4 hours, and the resultant was crushed
again with a ball mill to adjust the degree of aggregation, whereby a
source powder for the second layer compact was obtained. An intermediate
layer slurry prepared by adding an acryl-based binder and toluene to the
powder and blending the resultant was applied to the surface of the
formed first layer sintered body through the use of a screen printing
method to form a second layer compact film, and the resultant was fired
at the temperature shown in Table 1 for 3 hours.

[0141] Sample Nos. 12 to 15 shown in Table 1 were formed by firing the
stacked compact without forming the first layer and firing only the
second layer through a separate process. In Sample No. 16, the second
layer was not formed. In Sample Nos. 1 to 11 and Sample Nos. 17 to 22
having the first layer and the second layer formed thereon, the first
layer was denser than the second layer.

[0142] Thereafter, a section was observed by the use of a scanning
electron microscope and the separation of the first layer and the solid
electrolyte layer was checked. The thicknesses of the first layer and the
second layer were measured and are described in Table 1.

[0143] Regarding the fixing strength between the second layer and the
solid electrolyte layer or the first layer, the absence of fixing
strength was determined when the separation was caused by rubbing the
resultant with a finger or processing the resultant with an ultrasonic
cleaner, and the presence of fixing strength was determined when the
separation was not caused in any case.

[0144] A mixture solution including a
La0.6Sr0.4Co0.2Fe0.8O3 powder with an average
particle size of 2 μm and isopropyl alcohol was prepared, the mixture
solution was sprayed and applied to the surface of the second layer of a
stacked sintered compact to form an air electrode layer compact, the
resultant was baked in at 1100° C. for 4 hours to form an air
electrode layer, whereby the fuel cell shown in FIGS. 1(a) and 1(b) was
produced.

[0145] The size of the produced fuel cell was 25 mm×200 mm, the
thickness of the conductive support substrate (the distance between both
surfaces of the flat part n) was 2 mm, the open porosity thereof was 35%,
the thickness of the fuel electrode layer was 10 μm, the open porosity
thereof was 24%, the thickness of the air electrode layer was 50 μm,
the porosity thereof was 40%, and the relative density was 97%.

[0146] Hydrogen-containing gas was made to flow in the fuel cell and a
reduction process was performed on the conductive support substrate and
the fuel electrode layer at 850° C. for 10 hours.

[0147] In the obtained fuel cell, the maximum content of Y in the site of
the solid electrolyte layer within 1 μm from the interface thereof
with the intermediate layer and the content of Zr in the site where the
maximum content of Y was detected were measured through the use of the
STEM-EDS (quantitative analysis using scanning transmission electron
microscope-energy dispersive X-ray spectroscopy) quantitative analysis,
and the comparison results of the contents of the rare-earth element
excluding Ce in the first layer and the second layer and the values
obtained by dividing the contents of the rare-earth element excluding Ce
in the respective sites of the solid electrolyte layer by the content of
Zr are shown in Table 1. The ratio of Y and Zr in the site where the
maximum content of Y was detected in the table means the ratio of the
maximum content of Y in the site where the maximum content of Y was
detected in the site of the solid electrolyte layer within 1 μm from
the interface thereof with the intermediate layer and the content of Zr
in the site where the maximum content of Y was detected.

[0148] The contents of the rare-earth element excluding Ce in the first
layer and the second layer, the contents of the rare-earth element
excluding Ce in the intermediate layer at the sites of the solid
electrolyte layer, the site being 1 μm and 0.5 μm away from the
interface between the solid electrolyte layer and the intermediate layer
(the first layer or the second layer), and the content of Zr were
measured in the same way, and the comparison results of the contents of
the rare-earth element excluding Ce in the first layer and the second
layer and the values obtained by dividing the contents of the rare-earth
element excluding Ce in the respective sites of the solid electrolyte
layer by the content of Zr are shown in Table 1. The ratio of the
rare-earth element excluding Ce and Zr at the site of the solid
electrolyte layer, the site being 0.5 μm away from the interface
between the solid electrolyte layer and the intermediate layer means the
ratio of Y and Zr contained at the site of the solid electrolyte layer,
the site being 0.5 μm away from the interface between the solid
electrolyte layer and the intermediate layer.

[0149] By causing the sample thickness to be substantially uniform in
about 50 nm through the use of the FIB-micro sampling method during the
preparation of the STEM sample, the quantitative error was suppressed.

[0150] In the obtained fuel cell, the diffusion of Zr in the solid
electrolyte layer into the intermediate layer (the first layer and the
second layer) and the diffusion of Sr in the air electrode layer into the
solid electrolyte layer were surface-analyzed with an EPMA (X-ray micro
analyzer) and are described as presence or absence of Zr and Sr in Table
1.

[0151] Regarding the presence and absence of Zr and Sr, the absence was
determined when Zr was not present in the first layer and the second
layer or when Sr was not present in the solid electrolyte layer, and the
presence was determined when Zr or Sr was present therein.

[0152] Subsequently, fuel gas was made to flow in the fuel gas flow
channels of the obtained fuel cell, oxygen-containing gas was made to
flow outside the fuel cell, the fuel cell was heated to 600° C. by
the use of an electric furnace, the fuel cell was subjected to a power
generation test for 3 hours under the conditions of a fuel utilization of
75% and a current density of 0.3 A/cm2, and the power generation
performance (voltage) of the fuel cell at 600° C. is checked. The
result is shown in Table 1.

[0153] Thereafter, the fuel cell was heated to 750° C. by the use
of an electric furnace and was subjected to a power generation test for 3
hours under the conditions of a fuel utilization of 75% and a current
density of 0.3 A/cm2 and the power generation performance (voltage)
of the fuel cell at 750° C. was checked. The result is shown in
Table 1.

[0154] Then, the fuel cell was allowed to generate power at 750° C.
for 1000 hours under the conditions of a fuel utilization of 75% and a
current density of 0.6 A/cm2. At this time, the voltage after 1000
hours was measured using the value at a power generation time of 0 as an
initial voltage, and the variation from the initial voltage was
calculated as a decay rate, and the decay rate of the power generation
performance was calculated.

[0155] As to the evaluation of the decay of the power generation
performance, the decay rate of less than 0.5% was evaluated as extremely
small, the decay rate of 0.5% to 1% was evaluated as pretty small, the
decay rate of 1 to 3% was evaluated as small, the decay rate of 3 to 5%
was evaluated as large, and the decay rate of equal to or more than 5%
was evaluated as intense. The evaluation results are shown in Table 1.

[0156] From the results shown in Table 1, in Sample Nos. 1 to 20 in which
the stacked compact was calcined, it can be seen that the value obtained
by dividing the maximum content of Y in the site of the solid electrolyte
layer within 1 μm from the interface thereof with the intermediate
layer by the content of Zr in the site where the maximum content of Y was
detected was equal to or less than 0.25. The site having a large content
of Y did not appear in the site of the solid electrolyte layer within 1
μm from the interface thereof with the intermediate layer.

[0157] In addition, it can be seen that the voltage at 750° C. (mV)
was equal to or more than 750 mV and the power generation performance at
a high temperature was improved.

[0158] On the contrary, in Sample Nos. 21 to 23 in which the stacked
compact was not calcined, it can be seen that the value obtained by
dividing the maximum content of Y in the site of the solid electrolyte
layer within 1 μm from the interface thereof with the intermediate
layer by the content of Zr in the site where the maximum content of Y was
detected was equal to or more than 0.32. The site having a large content
of Y appears in the site of the solid electrolyte layer within 1 μm
from the interface thereof with the intermediate layer. The voltage at
750° C. (mV) was equal to or less than 650 mV.

[0159] From the results shown in Table 1, in Sample Nos. 1 to 11 in which
the intermediate layer was formed using CeO2 solid solution having a
rare-earth element excluding Ce as the source powder of the second layer
and using CeO2 (CeO2 solid solution having no rare-earth
element) as the source powder of the first layer, it can be seen that the
value obtained by dividing the content of the rare-earth element
excluding Ce in the intermediate layer by the content of Zr was equal to
or less than 0.1 at the site of the solid electrolyte layer, the site
being 0.5 μm away from the interface between the solid electrolyte
layer and the intermediate layer, the voltage at 600° C. (mV) was
equal to or more than 650 mV, and the power generation performance at a
low temperature was improved.

[0160] When the first layer and the solid electrolyte layer were co-fired
and the second layer was fired at a temperature lower by 200° C.
or higher than the co-firing temperature (Samples Nos. 1 to 9), it can be
seen that Zr did not diffuse into the second layer, the fixing strength
of the second layer was superior, Sr which is the component of the air
electrode layer 1 was not contained in the solid electrolyte layer, and
the deterioration in power generation performance was very small.

[0161] When the first layer and the second layer were made to be thicker
or thinner (Sample Nos. 7 to 9), it can be seen that Zr did not diffuse
into the second layer, the fixing strength of the second layer was
superior, Sr which is the component of the air electrode layer 1 was not
contained in the solid electrolyte layer, and the deterioration in power
generation performance was very small.

[0162] On the other hand, when the first layer was co-fired but the second
layer was fired at a temperature equal to or higher than 1400° C.,
that is, when the firing temperature of the second layer was lower than
the co-firing temperature but the temperature difference was lower than
200° C. (Sample Nos. 10 and 11), it can be seen that Sr which is
the component of the air electrode layer 1 was not contained in the solid
electrolyte layer, the fixing strength of the second layer was superior,
and the voltage at 600° C. (mV) was equal to or higher than 650
mV, but the diffusion of Zr into the second layer appeared and thus the
deterioration in power generation performance was large.

[0163] When the second layer was stacked on the sintered body without the
first layer (Sample Nos. 12 to 15) or when the second layer was not
formed (Sample No. 16), it can be seen that the value obtained by
dividing the content of the rare-earth element excluding Ce in the
intermediate layer by the content of Zr was more than 0.1 at the site of
the solid electrolyte layer, the site being 0.5 μm away from the
interface between the solid electrolyte layer and the intermediate layer,
the voltage at 600° C. (mV) was less than 650 mV, the power
generation performance at a low temperature was lowered, and the
deterioration in power generation performance was intense. In Sample Nos.
14 and 15, the fixing strength was present, but this is because the
fixing strength was enhanced due to Zr in the solid electrolyte layer and
Zr diffusing into the second layer.

[0164] In Sample Nos. 17 to 20 in which the first layer and the second
layer were formed using the source powder of CeO2 solid solution
having the rare-earth element excluding Ce, it can be seen that Zr did
not diffuse into the second layer, the fixing strength of the second
layer was superior, Sr which was the component of the air electrode layer
was not contained in the solid electrolyte layer, and the deterioration
in power generation performance was pretty small, but that the value
obtained by dividing the content of the rare-earth element excluding Ce
in the intermediate layer by the content of Zr was more than 0.1 at the
site of the solid electrolyte layer, the site being 0.5 μm away from
the interface between the solid electrolyte layer and the first layer,
the voltage at 600° C. (mV) was equal to or less than 600 mV, and
the power generation performance at a low temperature was lowered.